A Reason Why a Perfectly Competitive Firm's Demand for Labour

Multiple Choice

A reason why a perfectly competitive firm's demand for labour curve slopes downward is that

A) each additional unit of labour hired is less efficient than previously hired units.
B) in the short run, as more labour is hired, labour's marginal product falls because of the law of diminishing returns.
C) the extra cost of hiring additional units of labour increases as a firm hires more units of labour.
D) the firm's demand curve for the product that uses labour is downward sloping.